Rock mechanics - Characterization of rock masses

Rock mechanics is a discipline at the interface of mechanics and geology. It belongs to both the geosciences and the engineering sciences. Its application to engineering problems forms part of geotechnics, in the same way as soil mechanics. While many of the concepts developed in soil mechanics are applicable to rock mechanics, the differences between these two geomaterials imply differences in mechanical behavior and therefore the definition of rock-specific concepts, which this article, together with [C 350] , aims to highlight. When it comes to rock mechanics, we need to distinguish between rock (also referred to as rock matrix in this article) and rock mass, the latter being made up of discontinuities in addition to the rock matrix.

In this article, the point of view adopted is that of the application of rock mechanics within the framework of engineering projects, which as a general rule concerns the scale of the rock mass. The wording is therefore oriented towards the application of rock mechanics concepts to the rock mass as the object of study. Any project involving a rock mass requires the rock mass to be recognized and precisely characterized from a geotechnical point of view. A rock mass is a discontinuous medium made up of more or less intact blocks of rock, separated by discontinuities of various kinds. It is also a heterogeneous, anisotropic and multiphase medium, whose behavior is also affected by a strong scaling effect. The aim of this article is to present the essential concepts of rock mechanics, highlighting the specific mechanical properties of discontinuities and the rock mass.

The article is organized as follows. Firstly, a general description of the rock mass highlights the main characteristics and specific features of the rock mass that underlie its geomechanical behavior. The next two sections present the main elements of rock mass characterization. The second section is devoted to the description and characterization of discontinuities structuring the massif into boulders. The principles of discontinuity description and characterization of mechanical behavior are presented. The third section details the characterization of the rock mass, as an assembly of a rock matrix and a set of families of discontinuities. Topics covered include the characterization of discontinuity systems, which constitutes the structural description of the rock mass, geological, geophysical and geomechanical characterization on a mass scale, in situ stress determination, the importance of...